Airborne reconnaissance systems have been widely used for many years now, particularly for obtaining images from the air of areas of interest.
Originally, a film camera was used on board of the aircraft for capturing images of the terrain. The main problem of an airborne, film-camera based reconnaissance system is the length of time required for developing the film, an operation that can be performed only after landing. This problem has been overcome in more modern systems by the use of a one-dimensional vector or a two-dimensional array of light-sensitive sensors (generally such an array is called a “focal plane array” hereinafter also referred to as FPA) in the camera for obtaining electronic images that are then electronically stored within the aircraft, and/or transmitted to a ground base station. This is generally done in such systems by scanning the area of interest.
Airborne reconnaissance systems are generally used to obtain images of hostile areas, and therefore the task of obtaining such images involves some particular requirements, such as:    1. Flying the aircraft at high altitude and speeds in order to reduce the risk of being targeted by enemy weapons, and in order to widen the area captured by each image;    2. When carrying out reconnaissance in low height and/or high speed (i.e., a high V/H ratio), trying not to compromise the image quality.    3. Trying to capture as much relevant image information as possible during as short as possible flight time;    4. Trying to operate under various visibility conditions, while not compromising the resolution of the images and their quality.
The need for securing the reconnaissance aircraft, while flying above or close to hostile areas, has significantly increased flying costs and risks, as sometimes the reconnaissance mission requires escorting of the aircraft by other, fighter aircrafts. Therefore, the need for enabling a short and reliable mission is of a very high importance.
There are several other problems generally involved in carrying out airborne reconnaissance. For example, capturing images from a fast-moving aircraft introduces the need for the so-called Forward Motion Compensation (Hereinafter, the term “Forward Motion Compensation” will be shortly referred to as FMC), to compensate for aircraft movement during the opening of the camera shutter (whether mechanical or electronic; in the latter case, the opening of the camera shutter for the purpose of exposure is equivalent to the integration of light photons by the light-sensitive components).
When light-sensitive sensors are used in the camera, three major scanning types are used:    i. The Along-Track Scanning (also known as “push-broom scanning”)—In a first configuration of the Along-Track Scanning, the light-sensitive sensors are arranged in a one-dimensional vector (row), perpendicular to the flight direction. The scanning of the imaged area is obtained by the progression of the aircraft. In one specific configuration of Along-Track Scanning, generally called Along-Track TDI (Time Delayed Integration) configuration, a plurality of such parallel one-dimensional vectors (pixel-rows) perpendicular to the flight direction are provided at the focal plane forming a two-dimensional array. In that case, however, the first row of the array captures an area strip, while the subsequent rows are used to capture the same strip, but at a delay dominated by the aircraft progression. Then, for each row of pixels, a plurality of corresponding pixels of all the rows in the array, as separately measured, are first added, and then averaged in order to determine the pixel measured light intensity value. More particularly, each pixel in the image is measured N times (N being the number of rows) and then averaged. This Along-Track TDI configuration improves the signal-to-noise ratio.    ii. The Across-Track Scanning (also known as “Whiskbroom Scanning”)—In the Across-Track Scanning, a one-dimensional sensing vector of light-sensitive sensors, arranged parallel to the flight direction, is used. The camera including the sensing vector is positioned on gimbals having one degree of freedom, which, during the flight, repeatedly moves the camera right and left in a direction perpendicular to the direction of flight, while always keeping the vector in an orientation parallel to the direction of flight. Another Across-Track Scanning configuration uses a moving mirror or prism to sweep the line of sight (hereinafter, LOS) of a fixed vector of sensors across-track, instead of moving the whole camera. Another configuration of the Across-Track Scanning is the Across-Track TDI configuration. In this configuration there exists a plurality of vectors (columns) in a direction parallel to the flight direction, forming a two-dimensional array. This Across-Track TDI, in similarity to the Along-Track Scanning TDI, provides an improved reliability in the measuring of pixel values, more particularly, an improvement in the signal-to-noise ratio.    iii. Digital Framing Scanning: In Digital Framing Scanning, a two-dimensional array of light-sensitive sensors is positioned with respect to the scenery. In U.S. Pat. No. 5,155,597 and U.S. Pat. No. 6,256,057 the array is positioned such that its column-vectors (a column being a group of the array's columns) are parallel to the flight direction. Forward motion compensation (FMC) is provided electronically on-chip (in the detector focal plane array) by the transferring of charge from a pixel to the next adjacent pixel in the direction of flight during the sensor's exposure time (also called “integration time”). The charge transfer rate is determined separately for each column (or for the whole array as in U.S. Pat. No. 6,256,057 where a slit is moved in parallel to the columns direction), depending on its individual distance (range) from the captured scenery, assuming flat ground. In WO 97/42659 this concept is extended to handle transferring of charge separately for each cell instead of column, a cell being a rectangular group of pixels. In the system of U.S. Pat. No. 5,692,062, digital image correlation between successive frames captured by each column is performed, in order to measure the velocity of the scenery with respect to the array, and the correlation result is used for estimating the average range of each column to the scenery, for the purpose of motion compensation in terrain with large variations. This compensation method requires capturing of three successive frames for each single image, two for the correlation process and one for the final motion-compensated frame. The system of U.S. Pat. No. 5,668,593 uses a 3-axis sightline stepping mechanism for expanding coverage of the area of interest, and it applies a motion compensation technique by means of transferring of charge along columns. U.S. Pat. No. 6,130,705 uses a zoom lens that automatically varies the camera field of view based on passive range measurements obtained from digital image correlation as described above. The field of view is tuned in accordance with prior mission requirements for coverage and resolution.
A significant problem which is characteristic to all the above types of prior art reconnaissance systems is their limited field of view. Generally, the prior art systems comprise a lens at the front of the imaging system, which impinge the image onto a focal plane array through some more optical means. The lens generally has a limited field of view, in a typical range of up to 30°. Any attempt to increase the field of view results in a significant reduction in the resolution of the captured image. Therefore, when there is a need to obtain high resolution images in a wide field of regard with the prior art systems having a limited field of view, most such systems need a scanning mechanism, for repeatedly scanning the terrain perpendicular to the flight direction.
Another recent prior art reconnaissance system for a low altitude and wide coverage (about 180°), such as the one known as IRLS (Infra Red Line Scanner), uses a focal plane array which is fixed to the aircraft structure (not gimbaled). The terrain scanning perpendicular to the flight direction is made using a rotating prism which located in front of the camera.
Hereinafter, the term “field of regard” refers to the spatial section within which the camera line of sight can be directed without obscuration. In such a manner, the field of regard was increased up to 180°. However, such an approach requires a very expensive, heavy, and complicated mechanism. Moreover, such mechanical scanning could have been performed at a limited rate due to its structure, and in order to fully scan the area as required, the maximal flight velocity of the aircraft was limited. This limitation of the flight velocity is a very significant drawback, as reconnaissance missions are generally performed over enemy territory.
Still a significant drawback of reconnaissance systems of the prior art, for example, U.S. Pat. No. 5,155,597, U.S. Pat. No. 5,692,062, WO 97/42659, U.S. Pat. No. 6,130,705, and U.S. Pat. No. 6,256,057 is their limited ability to capture images in a wide range of a field of regard. Systems of the prior art sometimes use several dedicated systems for different sight directions (e.g. a separate system for down-looking, and others for side-oblique looking), which significantly increase the cost and weight of the whole reconnaissance system. The present invention provides to the aircraft the ability of capturing relatively high resolution images with single focal plane array, simultaneously from a wide field of regard (generally down and sideways of the aircraft) with no need for a mechanical scanning mechanism for changing the direction of sight.
It is therefore an object of the present invention to provide an airborne reconnaissance system capable of obtaining relatively high resolution and reliable images of the terrain, within a very wide field of regard.
It is still another object of the invention to eliminate the need for any mechanism for changing the direction of the camera line of sight, while still maintaining image capturing in a very wide field of regard, therefore increasing the reliability of the reconnaissance system.
It is still another object of the present invention to make the system of the present invention compact, enabling it to be accommodated within a limited volume compartment attached to the aircraft.
Other objects and advantages of the present invention will become apparent as the description proceeds.